Liquid crystal display apparatuses use a liquid crystal panel that uses the optical characteristic of a liquid crystal that changes its initial alignment orientation to another alignment state. Since the liquid crystal display apparatus can be driven by a lower voltage than conventional display apparatuses, it is suitable to be driven by LSIs, consumes low power, and allows its thickness and weight to be reduced. In recent years, in line with the trend of increasing the size of the screen and the capacity of the apparatus, the liquid crystal display apparatuses have been developed and commercialized so as to be mounted in OA or AV equipments.
Presently, depending on required features, relevant products include liquid crystal display apparatuses that use changes in the alignment state of the liquid crystal caused by the application of electric fields, that is, STN (super twisted nematic) liquid crystal display apparatuses based on a simple matrix method and using the electrooptical characteristic of the liquid crystal or TFT (thin film transistor) liquid crystal display apparatuses based on an active matrix method.
FIG. 8 shows a configuration drawing showing the basic structure of a liquid crystal display panel used for such a liquid crystal display apparatus. In this figure, 1 is a segment-side glass substrate; 2 is a common-side glass substrate; 3 is a sealing agent containing spacers; 4 is a common-side ITO (indium tin oxide) electrode formed of a transparent electrode film; 5 is a segment-side ITO electrode formed of a transparent electrode film; 6 is a vertical black matrix; 7 is an alignment layer; 8 is a spacer; 9 is a liquid crystal; and 10 is a color filter.
In this manner, the liquid crystal display panel used for the liquid crystal display apparatus has a sandwich structure in which the liquid crystal is sandwiched between the two glass substrates on which the transparent electrode pattern film is formed. A polymeric thin film is formed on the transparent electrode film to orient the liquid crystal. In the STN liquid crystal display panel, the cell thickness d between the substrates is normally 5 to 7 .mu.m. In this liquid crystal display panel, the polymeric thin film is rubbed to control the alignment of the liquid crystal so as to provide a pretilt angle of 3.degree. to 8.degree.. In addition, the STN method uses the birefringence of the liquid crystal and the optical rotatory of light and provides a significantly-steep-threshold characteristic by twisting the alignment direction of the liquid crystal between the two substrates through an angle of 180.degree. to 270.degree.. Thus, the cell thickness d requires an accuracy equivalent to 0.05 to 0.1 .mu.m according to the STN method.
In addition, since the liquid crystal molecules are twisted through the above angle within the predetermined thickness d, a trace amount of chiral agent is added to the liquid crystal to induce a specified twisted liquid crystal alignment. The trace amount of agent added depends on the alignment layer and pretilt angle, liquid crystal materials, the chiral agent, and the twist angle, and is normally 1 wt %.
The optical characteristics of the liquid crystal display panel are obtained by the birefringence and dielectric constant anisotropy .DELTA..di-elect cons. of the liquid crystal molecules. These optical characteristics vary depending on the composition of the liquid crystal materials including their viscosities and elastic constants or the adjustment of the composition ratio. Various liquid crystal materials are now developed and mixed together to provide desired characteristics.
.DELTA..di-elect cons. is set depending on the balance among the threshold voltage, drive waveform, cell thickness d, twist angle, and other optical characteristics taking into consideration the voltage resistance of LSIs that drive the liquid crystal display panel. The characteristics of the liquid crystal display panel are finally determined by the combination of .DELTA.n.times.d, that is, the product of the birefringence of the liquid crystal .DELTA.n and the thickness of the liquid crystal layer (cell thickness) d and the optical compensation configuration of an optical retardation film.
In the conventional liquid crystal display panel used for the STN liquid crystal display panel, in order to twist the nematic liquid crystal molecules sandwiched between the glass substrates through a predetermined twist angle, a chiral nematic liquid crystal or a cholesterol derivative is added as a chiral agent to induce a twisted alignment. Although, however, the chiral nematic liquid crystal is similar to the nematic liquid crystal in molecular shape and chemical and optical stability, the nature of the above chiral agent is inherently different from that of the nematic liquid crystal and causes various inconveniences in the liquid crystal display panel.
Specifically, due to its asymmetric carbon atoms of a high stereoscopic nature located at its molecular terminal group, the chiral agent obstructs the order of the alignment of the liquid crystal molecules in the liquid crystal bulk or interface to degrade the uniformity of the micro display. It also causes similar non-uniform display associated with the alignment of the spacers used to maintain the cell thickness d.
In addition, the addition of the chiral agent reduces the temperature (Tni point) changing from the nematic liquid crystal layer to an isotropic liquid to degrade the temperature characteristic, thereby forcing the characteristic balance (contrast and response speed) to be reduced to achieve the same temperature characteristic.
This invention solves these problems and provides a liquid crystal display panel that can improve alignment stability, display uniformity, and display quality.